dm: update test on of_offset in ofnode_valid

Update the test for node.of_offset because an invalid offset is not
always set to -1 because the return value of the libfdt functions are:
+ an error with a value < 0
+ a valid offset with value >=0

For example, in ofnode_get_by_phandle() function, we have:
node.of_offset = fdt_node_offset_by_phandle(gd->fdt_blob, phandle);
and this function can return -FDT_ERR_BADPHANDLE (-6).

Without this patch, the added test dm_test_ofnode_get_by_phandle failed.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/ofnode.h b/include/dm/ofnode.h
index d7852da..98c64fe 100644
--- a/include/dm/ofnode.h
+++ b/include/dm/ofnode.h
@@ -128,7 +128,7 @@
 	if (of_live_active())
 		return node.np != NULL;
 	else
-		return node.of_offset != -1;
+		return node.of_offset >= 0;
 }
 
 /**
diff --git a/test/dm/ofnode.c b/test/dm/ofnode.c
index 8bfb706..4ae8d28 100644
--- a/test/dm/ofnode.c
+++ b/test/dm/ofnode.c
@@ -19,6 +19,22 @@
 }
 DM_TEST(dm_test_ofnode_compatible, UT_TESTF_SCAN_PDATA | UT_TESTF_SCAN_FDT);
 
+static int dm_test_ofnode_get_by_phandle(struct unit_test_state *uts)
+{
+	/* test invalid phandle */
+	ut_assert(!ofnode_valid(ofnode_get_by_phandle(0)));
+	ut_assert(!ofnode_valid(ofnode_get_by_phandle(-1)));
+
+	/* test first valid phandle */
+	ut_assert(ofnode_valid(ofnode_get_by_phandle(1)));
+
+	/* test unknown phandle */
+	ut_assert(!ofnode_valid(ofnode_get_by_phandle(0x1000000)));
+
+	return 0;
+}
+DM_TEST(dm_test_ofnode_get_by_phandle, UT_TESTF_SCAN_PDATA | UT_TESTF_SCAN_FDT);
+
 static int dm_test_ofnode_by_prop_value(struct unit_test_state *uts)
 {
 	const char propname[] = "compatible";